1. Technical Field
The invention relates to a controller for controlling an air-fuel ratio based on an output of an exhaust gas sensor disposed in an exhaust system of an internal-combustion engine.
2. Description of the Related Art
A catalyst converter is provided in an exhaust system of an internal combustion engine of a vehicle. When the air-fuel ratio of air-fuel mixture introduced into the engine is lean, the catalyst converter oxidizes HC and CO with excessive oxygen included in the exhaust gas. When the air-fuel ratio is rich, the catalyst converter reduces Nox with HC and CO. When the air-fuel ratio is in the stoichiometric air-fuel ratio region, HC, CO and Nox are simultaneously and effectively purified.
An exhaust gas sensor is provided downstream of the catalyst converter. The exhaust gas sensor detects the concentration of oxygen included in the gas that is discharged into the exhaust system. Feedback control for the air-fuel ratio of the engine is performed based on the output of the exhaust gas sensor.
As an example of the feedback control for the air-fuel ratio, Japanese Patent Application Unexamined Publication No. H11-153051 proposes response assignment control in which a switching function is defined. This control converges the output of the exhaust gas sensor to a target value by converging the value of the switching function to zero. A controlled variable (a target air-fuel ratio) for converging the output of the exhaust gas sensor to the target value is calculated. The output of the exhaust gas sensor and the output of the air-fuel ratio (LAF) sensor, which is disposed upstream of the catalyst converter for detecting the air-fuel ratio, are used for the calculation of the controlled variable. A fuel amount to be supplied to the engine is controlled according to the calculated controlled variable.
Recently, there is a trend to enhance a response of the exhaust gas sensor so as to stabilize the accuracy of detecting deterioration of the catalyst and to decrease the amount of discharged NOx. When a response of the exhaust gas sensor is enhanced, high-frequency components, which are called chemical noise, may be introduced into the output of the exhaust gas sensor. Such chemical noise may cause variations in the target air-fuel ratio because the target air-fuel ratio is calculated based on the output of the exhaust gas sensor. Such variations in the target air-fuel ratio may cause a large variation in the actual air-fuel ratio, which reduces the purification rate of the catalyst.
The air-fuel ratio is sometimes made rich so as to protect the engine and the catalyst. Such enrichment of the air-fuel ratio increases the amount of discharged CO. In order to suppress the discharge of CO, it is preferable to perform the air-fuel ratio control in a form of closed loop. On the other hand, the air-fuel ratio control for making the air fuel ratio lean may be performed so as to improve the fuel efficiency. In such a state in which the air-fuel ratio is made lean, it is preferable to perform the air-fuel ratio control in the form of closed loop. In order to stably perform the closed-loop air-fuel ratio control, there is a trend to expand a detection range of the air-fuel ratio (LAF) sensor.
There is a limitation in the performance of an A/D converter that converts an analog signal from the air-fuel ratio sensor into a digital signal. When a detection range of the air-fuel ratio sensor is expanded, such limitation of the performance of the A/D converter reduces the resolution of the air-fuel ratio detected by the air-fuel ratio sensor. Such resolution reduction may reduce the capability to make the actual air-fuel ratio follow the target air-fuel ratio in the air-fuel ratio control, which reduces the purification rate of the catalyst. Such resolution reduction may also reduce the accuracy of identifying a model parameter for the air-fuel ratio control because the model parameter is identified based on the actual air-fuel ratio. The reduction of the accuracy of identifying a model parameter may also reduce the purification rate of the catalyst.
Therefore, there is a need for an apparatus and a method capable of removing chemical noise from the output of the exhaust gas sensor when a response of the exhaust gas sensor is enhanced. There is also a need for an apparatus and a method capable of compensating the shortage of resolution of the air-fuel ratio sensor when a detection range of the air-fuel ratio sensor is expanded.